The Genetics and Cell Biology of Fertilization
Total Page：16
File Type：pdf, Size：1020Kb
W&M ScholarWorks Arts & Sciences Articles Arts and Sciences 2011 The Genetics and Cell Biology of Fertilization Brian D. Geldziler Matthew R. Marcello Andrew Singson Diane C. Shakes William & Mary, [email protected] Follow this and additional works at: https://scholarworks.wm.edu/aspubs Recommended Citation Geldziler, B. D., Marcello, M. R., Shakes, D. C., & Singson, A. (2011). The genetics and cell biology of fertilization. In Methods in cell biology (Vol. 106, pp. 343-375). Academic Press. This Article is brought to you for free and open access by the Arts and Sciences at W&M ScholarWorks. It has been accepted for inclusion in Arts & Sciences Articles by an authorized administrator of W&M ScholarWorks. For more information, please contact [email protected]. NIH Public Access Author Manuscript Methods Cell Biol. Author manuscript; available in PMC 2012 February 8. NIH-PA Author ManuscriptPublished NIH-PA Author Manuscript in final edited NIH-PA Author Manuscript form as: Methods Cell Biol. 2011 ; 106: 343±375. doi:10.1016/B978-0-12-544172-8.00013-X. The Genetics and Cell Biology of Fertilization Brian D. Geldziler1, Matthew R. Marcello1, Diane C. Shakes2,3, and Andrew Singson1,3 1Waksman Institute, Rutgers University, Dept. of Microbiology and Molecular Genetics 2College of William and Mary, Department of Biology I. General Introduction/Background Successful fertilization is fundamentally important to a sexually reproducing species and requires a series of well-coordinated events including gamete activation, recognition, signaling, adhesion and fusion (Wassarman 1999; Singson, Zannoni et al. 2001; Primakoff and Myles 2002). Although our current understanding of these processes comes largely from work in marine invertebrates and vertebrate model systems, C. elegans has emerged as another powerful system for fertilization studies (Singson 2001; Yamamoto, Kosinski et al. 2006; Singson, Hang et al. 2008; Nishimura and L’Hernault 2010). Fertilization in C. elegans takes place in the spermatheca, the site of sperm storage in the hermaphrodite. The hermaphrodite reproductive tract consists of a bi-lobed gonad (Fig. 1A) in which a separate spermatheca connects each lobe to the shared uterus (the male gonad is single-lobed [Fig. 1B]). Within the hermaphrodite gonad, both gamete types are produced in a sequential manner. Sperm are produced first during the last larval stage of development and stored in the spermatheca. The gonad switches to oocyte production in the adult hermaphrodite. Oocytes undergo meiotic maturation as they move towards the uterus and are ovulated into the spermatheca, where they immediately contact multiple spermatozoa. Blocks to polyspermy exist, as only a single sperm fertilizes each oocyte (Parry, Velarde et al. 2009). The coordination of events leading to sperm/oocyte contact ensures highly efficient sperm utilization as virtually all functional sperm fertilize oocytes (Ward and Carrel 1979; Kadandale and Singson 2004). The hermaphrodite’s own sperm can be supplemented by mating to males; male sperm are deposited in the uterus and immediately travel to the spermatheca to await oocyte passage. A sperm-sensing mechanism ensures that metabolically costly oocytes are not wasted; when hermaphrodites lack sperm, they ovulate at a very low basal level. Conversely, the presence of sperm within the hermaphrodite spermatheca causes a dramatic increase in ovulation rate (McCarter, Bartlett et al. 1999) (Miller, Nguyen et al. 2001). After fertilization, the zygote secretes a multi-layered egg shell and begins embryonic development. Eggs then pass through the uterus and are laid before hatching (Fig. 2). C. elegans offers several advantages over other model systems for studying fertilization. Although its amoeboid sperm possess neither a flagellum nor an acrosome (Fig. 3), these sperm successfully perform the same tasks required of all spermatozoa (e.g. migration to the fertilization site, species-specific oocyte recognition, fusion). In addition, the events of fertilization can be directly observed in living animals through the worm’s transparent cuticle (McCarter, Bartlett et al. 1999). It is also possible to isolate large quantities of sperm and oocytes, though this is more challenging to do than in some other model systems (L’Hernault and Roberts 1995; Aroian, Field et al. 1997; Miller 2006). Fertilization studies 3Corresponding Authors ([email protected] and [email protected]). Geldziler et al. Page 2 in C. elegans routinely use molecular and genetic tools that are unavailable or difficult to use in other systems. The complete sequencing of the worm genome and the availability of NIH-PA Author Manuscript NIH-PA Author Manuscriptmicroarrays NIH-PA Author Manuscript greatly simplifies the identification and analysis of genes required for fertility (Singson 2001; Singson, Hang et al. 2008). Perhaps the greatest advantage of C. elegans is the ease with which one can perform forward genetics to screen for fertilization-defective mutants (discussed below). Such screens have identified many of these mutants, which may be classified broadly into those mutations affecting sperm (spe or fer mutants, for spermatogenesis or fertilization defective) and those affecting eggs/oocytes (egg mutants). Note that the fer designation has been discontinued and all new sperm development or function mutants are now given the spe designation. Although the majority of characterized spe/fer mutations affect sperm development, a subset of these mutations specifically affect sperm function (i.e. fertilization). spe-9 class mutants, for example, produce sperm that are unable to fertilize oocytes despite exhibiting normal morphology, motility and gamete contact (L’Hernault, Shakes et al. 1988; Singson, Mercer et al. 1998; Xu and Sternberg 2003; Chatterjee, Richmond et al. 2005; Kroft, Gleason et al. 2005). Recent studies have also uncovered egg mutations that specifically influence fertilization and/or egg activation (Kadandale, Stewart-Michaelis et al. 2005). A partial listing of characterized genes required for fertilization is given in Table 1. Many of the experimental tools and techniques discussed in this chapter were developed for the study of these genes. C. elegans also enables the evolutionary assessment of fertilization molecules and can help elucidate major molecular themes. For example, EGF-repeat-containing molecules have been implicated in fertilization across a wide evolutionary spectrum, from HrVC70 in ascidians (Sawada, Tanaka et al. 2004) to SPE-9 in worms (Singson, Mercer et al. 1998) and SED-1 in mammals (Ensslin and Shur 2003). In this chapter we introduce the major experimental approaches/implications to consider when using C. elegans to study fertilization. A general scheme for the study of sterility mutants in C. elegans is presented in Fig. 4. Detailed protocols can be found in the original literature and in L’Hernault and Roberts (1995). II. Finding Sterile Mutants A major advantage of using C. elegans for fertilization study is the ability to use forward genetic screens to isolate fertility mutants. Mutant screening in C. elegans is typically carried out using ethyl methane sulphonate (EMS) to induce mutations in the germline of wild-type hermaphrodites. In the subsequent F2 generation, homozygous mutants are screened for the phenotype of interest (L’Hernault, Shakes et al. 1988). Using standard concentrations of EMS, investigators can expect to find one null mutation per 2,000 gene copies examined (Jorgensen and Mango 2002) and approximately one in every 30 F2 generation animals will display a Spe phenotype (S. L’Hernault, personal communication). The power of these genetic screens lies in the ability to isolate mutations in sperm-specific genes by simply selecting hermaphrodites that are self-sterile but whose oocytes can be still be fertilized by wild-type male sperm (Fig. 5). More than 60 sperm-specific mutants have been identified this way, enabling the genetic delineation of sperm development and function pathways (L’Hernault, Shakes et al. 1988; L’Hernault 1997; L’Hernault 2006; Nishimura and L’Hernault 2010). C. elegans is particularly amenable to such sperm-specific screens because they can be carried out with hermaphrodites (rather than in males as in gonochoristic systems such as mice or flies) and thus remain independent of confounding issues such as male mating behavior or secondary sex characteristics (L’Hernault, Shakes et al. 1988; Lessard, Pendola et al. 2004; Wakimoto, Lindsley et al. 2004). Methods Cell Biol. Author manuscript; available in PMC 2012 February 8. Geldziler et al. Page 3 Genetic screens for conditional egg-sterile mutants have likewise yielded many interesting candidate genes. (G. Singaravelu, D. Shakes and A. Singson, unpublished). These NIH-PA Author Manuscript NIH-PA Author Manuscriptconditional NIH-PA Author Manuscript screens are especially useful for the isolation of fertility mutants as they allow the propagation of homozygous mutations in fertility genes. Isolating egg sterile mutants in standard non-conditional F2 screens requires considerably more work since progeny cannot be recovered from homozygous egg-sterile mutants. As a result, the mutant chromosome must be recovered from heterozygous siblings. Conditional fertility screens are similar to the conditional maternal effect lethal (mel) screens conducted in C. elegans (O’Connell, Leys et al. 1998; Jorgensen and Mango 2002). In a typical mel screen, worms with a pre-existing mutation that blocks normal egg-laying